This invention relates to the production of a mixture of hydrogen and nitrogen from solid carbonaceous materials and air. Ammonia synthesis gas is such a mixture having a molecular ratio of about 3 hydrogen to 1 nitrogen.
Ammonia synthesis gas is generally prepared by two-stage steam reforming of natural gas, with air in the correct proportion being introduced between stages. The air is introduced to provide nitrogen; however, combustion of part of the synthesis gas reduces the heat necessary to drive the endothermic reforming reaction to completion. This system produces an intermediate synthesis gas containing hydrogen, carbon monoxide and nitrogen, with the carbon monoxide plus hydrogen being approximately three times the moles of nitrogen. This gas is then catalytically reacted with steam in the so-called water gas shift reaction. In this well-known reaction, carbon monoxide reacts mole for mole with steam to produce one mole of hydrogen per mole of carbon monoxide. Thus, in a hydrogen-carbon monoxide mixture, carbon monoxide can be viewed as "potential" hydrogen. After the shift reaction, the gas is processed to remove water, carbon dioxide, and any traces of sulfur which may have been introduced in the hydrocarbon feed, resulting in a gas containing only hydrogen and nitrogen in the desired ratio.
With the decreasing reserves of natural gas, it has become apparent that other carbonaceous raw materials will ultimately be used in the manufacture of ammonia, methanol, and other synthesis gas derived products. Coal can be reacted with oxygen and steam to produce a mixture of hydrogen and carbon monoxide with minor amounts of carbon dioxide and sulfur compounds. If ammonia synthesis is the object of the gasification, the mixture is shifted and purified to produce pure hydrogen, after which an appropriate amount of nitrogen derived from air separation is blended with the purified hydrogen. A number of physical systems have been proposed to effect oxygen fueled gasification; however, generally they require an air separation plant or a purchased supply of pure oxygen. This adds to the capital cost and operating complexity; or, if purchased oxygen is used, the raw material costs chargeable to the product are greatly increased.
U.S. Pat. No. 2,795,559 discloses the production of a gas mixture of 3 volumes of hydrogen per volume of nitrogen by oxidatively gasifying a carbonaceous fuel, such as coal, with air as the sole source of oxygen at an elevated temperature and pressure to produce a synthesis gas containing a mixture of carbon monoxide, hydrogen and nitrogen, the latter being present in greater than one volume of nitrogen per three volumes of hydrogen. The carbon monoxide in said mixture is then reacted with steam, i.e., water gas shift reaction, to convert the carbon monoxide to carbon dioxide with the concomitant production of hydrogen. The carbon dioxide and water vapor are removed from the mixture leaving essentially a hydrogen and nitrogen mixture. The hydrogen and nitrogen mixture is then further refined by a series of steps wherein nitrogen in excess of 1 volume per 3 volumes of hydrogen is condensed at cryogenic conditions and removed to obtain the ammonia syn gas mixture. It is noted that the process described in this patent utilizes air in the gasification of the coal as the only source of oxygen and the reaction of air with the fuel, coal, is exothermic to maintain the gasification autogenously and supply heat for the endothermic water gas shift reaction.
U.S. Pat. No. 2,276,343 describes a continuous process for gasifying lignite to produce hydrogen by preheating a lignite containing 25-40% water to drive off water-containing vapor and render the lignite highly reactive. The mixture of highly reactive lignite and water vapor are reacted at an elevated temperature to produce a hydrogen rich gaseous mixture which is then separated from the lignite and purified. The lignite is heated and maintained at 500.degree. C. to 850.degree. C. in the described process by external means. External steam may be introduced to the reaction to produce additional hydrogen.
U.S. Pat. No. 3,620,697 discloses a process for producing hydrogen by reacting carbon, such as coal, with water utilizing a circulating inert particulate material as the heat source for the endothermic reaction. The inert particulate material is heated by burning a carbonaceous material therewith while being conveyed to the reaction. Similarly, U.S. Pat. No. 3,968,052 and U.S. Pat. No. 3,850,839 disclose processes for gasifying coal-derived char with steam in a fluid bed system where the heat for the endothermic gasification reaction is supplied by circulating through the system inert pellets which are heated by burning fines from the gasifier in their presence externally of the gasifier.